The NAVSTAR Global Positioning System or "GPS" was put into place by the government of the United States and it uses 24 satellites which transmit signals L.sub.1 and L.sub.2 at two different frequencies. These signals have respective center frequencies f.sub.1 =1575.42 MHz and f.sub.2 =1227.6 MHz, and they are generated by an onboard atomic clock operating at a frequency of 10.23 MHz.
If it is desired to compensate for effects due to propagation through the dispersive medium as constituted by the ionosphere, it is essential to have both the L.sub.1 and the L.sub.2 signals available simultaneously, and this is therefore essential in all applications which require high accuracy.
The L.sub.1 signal is modulated in phase quadrature by two pseudo-random codes, known as the clear/acquisition (C/A) code at a rate of 1.023 MHz and by a higher-frequency second code at a rate of 10.23 MHz. The L.sub.2 signal is modulated with a second code only, which second code is identical to that used for the L.sub.1 signal. The C/A code is public, however the second code is used in two modes at the discretion of the operator, namely:
either in the form of a public code known as P code; PA1 or else in the form of a security code that is secret, or difficult to obtain, known as the Y code. PA1 a) in separate systems for each of the signals L.sub.1 and L.sub.2 correlation is performed with a locally-generated replica of the P code, respectively in a first system for processing the signal L.sub.1 and in a second system for processing the signal L.sub.2, thereby producing respective first and second correlation signals; PA1 b) the first and second correlation signals are integrated over a period equal to an estimated period for one bit of the W code to obtain respective first and second W code bit signals; and PA1 c) the first and/or second W code bit signals are cross-correlated with the second and/or first correlation signals respectively, wherein,
The nominal operating mode of the GPS system implements the Y code. The reason for this is that the GPS operators desire to ensure that detection is not disturbed by any decoy signals which might be broadcast, in particular during military operations.
In other words, and contrary to certain opinions, the purpose of the Y code is not to degrade the performance of the GPS system for non-approved users, but to guarantee performance of the system for approved military users.
In addition, having access to the Y code in any event implies that new coding must be performed every year, which is not very practical, in particular for space-borne applications.
In order to enable civilian applications to be implemented without using the Y code, companies and research institutes have developed code-less tracking methods which make it possible to determine the L.sub.2 code and the carrier phase information with good accuracy. This situation has been recognized by the government of the United States which recently declared that the L.sub.2 signal can continue to be used in civilian applications, but solely for the purpose of performing accurate measurements of carrier phase. In other words, in the GPS system, signal modulation is going to remain compatible with a code-less tracking method.
Code-less tracking methods are particularly advantageous in the context of scientific applications such as geodesics, measuring movements of the Earth's crust, and meteorology, and also for determining the integrated value of the water vapor content of the troposphere, and they are presently in common use in precision receivers available on the market. In the above-mentioned applications, the main purpose is to measure the phase of the L.sub.2 carrier signal so as to be able to perform a correction that takes account of propagation phenomena through the ionosphere.
Code-less tracking methods are known in particular from U.S. Pat. No. 5,134,407 (ASHTECH TELESIS) and U.S. Pat. No. 5,541, 606 (TRIMBLE).
The method described in U.S. Pat. No. 5,134,407 relies on the fact that the Y code is, in fact, the modulo-2 sum of the known P code at a rate of 10.23 MHz, plus an encrypting code generally referred to as the A code or indeed as the W code, at a rate which is considerably slower. The exact form of the W code is, naturally, not known, however it is known that its rate is about 1/20th the rate of the P code, i.e. about 500 kHz.
The samples of the L.sub.1 and L.sub.2 signals are correlated with locally-generated replicas of the P code. The P code generators, which comprise two independent generators, or one generator together with a delay line, are controlled by a digitally controlled oscillator DCO which is in turn controlled for carrier tracking purposes by a microprocessor in a conventional phase-locked loop configuration, e.g. a Costas loop. The local P code is offset in time to be "aligned" with the input signal in order to obtain maximum energy. Once the P code replica has been correlated, the signal is filtered to reduce noise before estimating bits of the W code. This filtering is performed in a conventional manner by integration and storage, and the integration period is equal to the estimated duration T.sub.1 of one bit of W code. The consequence of unknown W code being present is thus that the predictive passband cannot be reduced below the bit rate of the W code, and that the signal-to-noise ratio S/N of the predetection band is very low.
Thermal noise on the L.sub.1 and L.sub.2 signals is statistically independent. This is used to decide on the sign of the W code bit in each of the two systems, L.sub.1 and L.sub.2, and to apply the result to the other system. This cross-correlation method makes it possible to use an integration period T.sub.2 that is longer than T.sub.1, thereby decreasing noise and increasing the post-detection signal/noise ratio. It is possible mathematically to determine the probability of a wrong decision being made in a predetection passband containing a high level of noise, if the ratio SNR is known for the signal power of the transmitted signal carries over the noise power for a unit passband. It can be deduced therefrom that a certain number of decisions will be wrong. Given that a wrong decision cancels a right decision, the technique can operate effectively only if the probability of a bit of the W code being detected correctly is significantly greater than 50%.